Filter using lithium niobate and lithium tantalate transversely-excited film bulk acoustic resonators

ABSTRACT

Acoustic filters are disclosed. A bandpass filter has a passband between a lower band edge and an upper band edge. The bandpass filter includes a plurality of transversely-excited film bulk acoustic resonators (XBARs) connected in a ladder filter circuit. The plurality of XBARs includes at least one lithium tantalate XBAR and at least one lithium niobate XBAR.

RELATED APPLICATION INFORMATION

This patent is a continuation of application Ser. No. 17/070,694, filedOct. 14, 2020, entitled FILTER USING LITHIUM NIOBATE AND LITHIUMTANTALATE TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATORS, whichclaims priority from provisional patent application 63/040,440, filedJun. 17, 2020, entitled FILTER USING LITHIUM NIOBATE AND LITHIUMTANTALATE XBARS. This application is incorporated herein by reference.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic waveresonators, and specifically to filters for use in communicationsequipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to passsome frequencies and to stop other frequencies, where “pass” meanstransmit with relatively low signal loss and “stop” means block orsubstantially attenuate. The range of frequencies passed by a filter isreferred to as the “pass-band” of the filter. The range of frequenciesstopped by such a filter is referred to as the “stop-band” of thefilter. A typical RF filter has at least one pass-band and at least onestop-band. Specific requirements on a passband or stop-band depend onthe specific application. For example, a “pass-band” may be defined as afrequency range where the insertion loss of a filter is better than adefined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be definedas a frequency range where the rejection of a filter is greater than adefined value such as 20 dB, 30 dB, 40 dB, or greater depending onapplication.

RF filters are used in communications systems where information istransmitted over wireless links. For example, RF filters may be found inthe RF front-ends of cellular base stations, mobile telephone andcomputing devices, satellite transceivers and ground stations, IoT(Internet of Things) devices, laptop computers and tablets, fixed pointradio links, and other communications systems. RF filters are also usedin radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for eachspecific application, the best compromise between performance parameterssuch as insertion loss, rejection, isolation, power handling, linearity,size and cost. Specific design and manufacturing methods andenhancements can benefit simultaneously one or several of theserequirements.

Performance enhancements to the RF filters in a wireless system can havebroad impact to system performance. Improvements in RF filters can beleveraged to provide system performance improvements such as larger cellsize, longer battery life, higher data rates, greater network capacity,lower cost, enhanced security, higher reliability, etc. Theseimprovements can be realized at many levels of the wireless system bothseparately and in combination, for example at the RF module, RFtransceiver, mobile or fixed sub-system, or network levels.

The desire for wider communication channel bandwidths will inevitablylead to the use of higher frequency communications bands. The currentLTE™ (Long Term Evolution) specification defines frequency bands from3.3 GHz to 5.9 GHz. These bands are not presently used. Future proposalsfor wireless communications include millimeter wave communication bandswith frequencies up to 28 GHz.

High performance RF filters for present communication systems commonlyincorporate acoustic wave resonators including surface acoustic wave(SAW) resonators, bulk acoustic wave (BAW) resonators, film bulkacoustic wave resonators (FBAR), and other types of acoustic resonators.However, these existing technologies are not well-suited for use at thehigher frequencies proposed for future communications networks.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view and two schematic cross-sectional viewsof a transversely-excited film bulk acoustic resonator (XBAR).

FIG. 2 is an expanded schematic cross-sectional view of a portion of theXBAR of FIG. 1.

FIG. 3 is an alternative schematic cross-sectional view of the XBAR ofFIG. 1.

FIG. 4 is a graphic illustrating a shear primary acoustic mode in anXBAR.

FIG. 5 is a schematic block diagram of a filter using XBARs.

FIG. 6 is a schematic cross-sectional view of two XBARs illustrating afrequency-setting dielectric layer.

FIG. 7 is a schematic diagram of a filter using lithium niobate andlithium tantalate XBARs.

FIG. 8 is a graph of the input/output transfer function of the filter ofFIG. 7 and the admittances of four shunt resonators from the filter ofFIG. 7.

FIG. 9 is a graph of the input/output transfer function of the filter ofFIG. 7 at two temperatures.

FIG. 10 is a schematic diagram of another filter using lithium niobateand lithium tantalate XBARs.

FIG. 11 is a graph of the input/output transfer function of the filterof FIG. 10 at two temperatures.

FIG. 12 is a schematic diagram of another filter using lithium niobateand lithium tantalate XBARs.

FIG. 13A is a schematic plan view of a filter using lithium niobate andlithium tantalate XBARs.

FIG. 13B is a schematic side view of the filter using lithium niobateand lithium tantalate XBARs of FIG. 13A.

FIG. 14 is a flow chart of a process for fabricating XBARs.

FIG. 15 is a flow chart of a process for fabricating a filter usinglithium niobate and lithium tantalate XBARs.

Throughout this description, elements appearing in figures are assignedthree-digit or four-digit reference designators, where the two leastsignificant digits are specific to the element and the one or two mostsignificant digit is the figure number where the element is firstintroduced. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously-described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

Transversely-excited film bulk acoustic resonators (XBARs) wereinitially described in U.S. Pat. No. 10,491,192. FIG. 1 shows asimplified schematic top view and orthogonal cross-sectional views of anXBAR 100. XBAR resonators such as the resonator 100 may be used in avariety of RF filters including band-reject filters, band-pass filters,duplexers, and multiplexers. XBARs are well suited for use in filtersfor communications bands with frequencies above 3 GHz.

The XBAR 100 is made up of a thin film conductor pattern formed on asurface of a piezoelectric plate 110 having a front surface 112 and aback surface 114. The front and back surfaces are essentially parallel.“Essentially parallel” means parallel to the extent possible withinnormal manufacturing tolerances. The piezoelectric plate is a thinsingle-crystal layer of a piezoelectric material such as lithiumniobate, lithium tantalate, lanthanum gallium silicate, gallium nitride,or aluminum nitride. The piezoelectric plate is cut such that theorientation of the X, Y, and Z crystalline axes with respect to thefront and back surfaces is known and consistent. In the examplespresented in this patent, the piezoelectric plates are Z-cut, which isto say the Z axis is normal to the front surface 112 and back surface114. However, XBARs may be fabricated on piezoelectric plates with othercrystallographic orientations including rotated Z-cut and rotatedYX-cut.

The back surface 114 of the piezoelectric plate 110 is attached to asurface 122 of the substrate 120 except for a portion of thepiezoelectric plate 110 that forms a diaphragm 115 spanning a cavity 140formed in the substrate 120. The portion of the piezoelectric plate thatspans the cavity is referred to herein as the “diaphragm” due to itsphysical resemblance to the diaphragm of a microphone. As shown in FIG.1, the diaphragm 115 is contiguous with the rest of the piezoelectricplate 110 around all of a perimeter 145 of the cavity 140. In thiscontext, “contiguous” means “continuously connected without anyintervening item”.

The substrate 120 provides mechanical support to the piezoelectric plate110. The substrate 120 may be, for example, silicon, sapphire, quartz,or some other material or combination of materials. The back surface 114of the piezoelectric plate 110 may be attached to the substrate 120using a wafer bonding process. Alternatively, the piezoelectric plate110 may be grown on the substrate 120 or otherwise attached to thesubstrate. The piezoelectric plate 110 may be attached directly to thesubstrate or may be attached to the substrate 120 via one or moreintermediate material layers.

The cavity 140 is an empty space within a solid body of the resonator100. The cavity 140 may be a hole completely through the substrate 120(as shown in Section A-A and Section B-B) or a recess in the substrate120 (as shown subsequently in FIG. 3). The cavity 140 may be formed, forexample, by selective etching of the substrate 120 before or after thepiezoelectric plate 110 and the substrate 120 are attached.

The conductor pattern of the XBAR 100 includes an interdigitaltransducer (IDT) 130. An IDT is an electrode structure for convertingbetween electrical and acoustic energy in piezoelectric devices. The IDT130 includes a first plurality of parallel elongated conductors,commonly called “fingers”, such as finger 136, extending from a firstbusbar 132. The IDT 130 includes a second plurality of fingers extendingfrom a second busbar 134. The first and second pluralities of parallelfingers are interleaved. The interleaved fingers overlap for a distanceAP, commonly referred to as the “aperture” of the IDT. Thecenter-to-center distance L between the outermost fingers of the IDT 130is the “length” of the IDT.

The term “busbar” refers to the conductors that interconnect the firstand second sets of fingers in an IDT. As shown in FIG. 1, each busbar132, 134 is an elongated rectangular conductor with a long axisorthogonal to the interleaved fingers and having a length approximatelyequal to the length L of the IDT. The busbars of an IDT need not berectangular or orthogonal to the interleaved fingers and may havelengths longer than the length of the IDT.

The first and second busbars 132, 134 serve as the terminals of the XBAR100. A radio frequency or microwave signal applied between the twobusbars 132, 134 of the IDT 130 excites a primary acoustic mode withinthe piezoelectric plate 110. As will be discussed in further detail, theprimary acoustic mode is a bulk shear mode where acoustic energypropagates along a direction substantially orthogonal to the surface ofthe piezoelectric plate 110, which is also normal, or transverse, to thedirection of the electric field created by the IDT fingers. Thus, theXBAR is considered a transversely-excited film bulk wave resonator.

The IDT 130 is positioned on the piezoelectric plate 110 such that atleast the fingers of the IDT 130 are disposed on the diaphragm 115 ofthe piezoelectric plate that spans, or is suspended over, the cavity140. As shown in FIG. 1, the cavity 140 has a rectangular shape with anextent greater than the aperture AP and length L of the IDT 130. Acavity of an XBAR may have a different shape, such as a regular orirregular polygon. The cavity of an XBAR may more or fewer than foursides, which may be straight or curved.

For ease of presentation in FIG. 1, the geometric pitch and width of theIDT fingers is greatly exaggerated with respect to the length (dimensionL) and aperture (dimension AP) of the XBAR. An XBAR for a 5G device willhave more than ten parallel fingers in the IDT 110. An XBAR may havehundreds, possibly thousands, of parallel fingers in the IDT 110.Similarly, the thickness of the fingers in the cross-sectional views isgreatly exaggerated in the drawings.

FIG. 2 shows a detailed schematic cross-sectional view of the XBAR 100.The piezoelectric plate 110 is a single-crystal layer of piezoelectricalmaterial having a thickness ts. ts may be, for example, 100 nm to 1500nm. When used in filters for LTE™ bands from 3.4 GHZ to 6 GHz (e.g.bands 42, 43, 46), the thickness ts may be, for example, 200 nm to 1000nm.

A front-side dielectric layer 214 may be formed on the front side of thepiezoelectric plate 110. The “front side” of the XBAR is the surfacefacing away from the substrate. The front-side dielectric layer 214 hasa thickness tfd. The front-side dielectric layer 214 is formed betweenthe IDT fingers 238. Although not shown in FIG. 2, the front sidedielectric layer 214 may also be deposited over the IDT fingers 238. Aback-side dielectric layer 216 may be formed on the back side of thepiezoelectric plate 110. The back-side dielectric layer 216 has athickness tbd. The front-side and back-side dielectric layers 214, 216may be a non-piezoelectric dielectric material, such as silicon dioxideor silicon nitride. tfd and tbd may be, for example, 0 to 500 nm. tfdand tbd are typically less than the thickness ts of the piezoelectricplate. tfd and tbd are not necessarily equal, and the front-side andback-side dielectric layers 214, 216 are not necessarily the samematerial. Either or both of the front-side and back-side dielectriclayers 214, 216 may be formed of multiple layers of two or morematerials.

The IDT fingers 238 may be one or more layers of aluminum, asubstantially aluminum alloy, copper, a substantially copper alloy,beryllium, gold, molybdenum, or some other conductive material. Thin(relative to the total thickness of the conductors) layers of othermetals, such as chromium or titanium, may be formed under and/or overthe fingers to improve adhesion between the fingers and thepiezoelectric plate 110 and/or to passivate or encapsulate the fingers.The busbars (132, 134 in FIG. 1) of the IDT may be made of the same ordifferent materials as the fingers. As shown in FIG. 2, the IDT fingers238 have rectangular cross-sections. The IDT fingers may have some othercross-sectional shape, such as trapezoidal.

Dimension p is the center-to-center spacing or “pitch” of the IDTfingers, which may be referred to as the pitch of the IDT and/or thepitch of the XBAR. Dimension w is the width or “mark” of the IDTfingers. The IDT of an XBAR differs substantially from the IDTs used insurface acoustic wave (SAW) resonators. In a SAW resonator, the pitch ofthe IDT is one-half of the acoustic wavelength at the resonancefrequency. Additionally, the mark-to-pitch ratio of a SAW resonator IDTis typically close to 0.5 (i.e., the mark or finger width is aboutone-fourth of the acoustic wavelength at resonance). In an XBAR, thepitch p of the IDT is typically 2 to 20 times the width w of thefingers. In addition, the pitch p of the IDT is typically 2 to 20 timesthe thickness ts of the piezoelectric slab 110. The width of the IDTfingers in an XBAR is not constrained to one-fourth of the acousticwavelength at resonance. For example, the width of XBAR IDT fingers maybe 500 nm or greater, such that the IDT can be fabricated using opticallithography. The thickness tm of the IDT fingers may be from 100 nm toabout equal to the width w. The thickness of the busbars (132, 134 inFIG. 1) of the IDT may be the same as, or greater than, the thickness tmof the IDT fingers.

FIG. 3 is an alternative cross-sectional view along the section planeA-A defined in FIG. 1. In FIG. 3, a piezoelectric plate 310 is attachedto a substrate 320. A portion of the piezoelectric plate 310 forms adiaphragm 315 spanning a cavity 340 in the substrate. The cavity 340does not fully penetrate the substrate 320. Fingers of an IDT aredisposed on the diaphragm 315. The cavity 340 may be formed, forexample, by etching the substrate 320 before attaching the piezoelectricplate 310. Alternatively, the cavity 340 may be formed by etching thesubstrate 320 with a selective etchant that reaches the substratethrough one or more openings (not shown) provided in the piezoelectricplate 310. In this case, the diaphragm 315 may be contiguous with therest of the piezoelectric plate 310 around a large portion of aperimeter of the cavity 340. For example, the diaphragm 315 may becontiguous with the rest of the piezoelectric plate 310 around at least50% of the perimeter of the cavity 340.

FIG. 4 is a graphical illustration of the primary acoustic mode ofinterest in an XBAR. FIG. 4 shows a small portion of an XBAR 400including a piezoelectric plate 410 and three interleaved IDT fingers430. A radio frequency (RF) voltage is applied to the interleavedfingers 430. This voltage creates a time-varying electric field betweenthe fingers. The direction of the electric field is primarily lateral,or parallel to the surface of the piezoelectric plate 410, as indicatedby the arrows labeled “electric field”. Since the dielectric constant ofthe piezoelectric plate is significantly higher than the surroundingair, the electric field is highly concentrated in the plate relative tothe air. The lateral electric field introduces shear deformation, andthus strongly excites a shear-mode acoustic mode, in the piezoelectricplate 410. Shear deformation is deformation in which parallel planes ina material remain parallel and maintain a constant distance whiletranslating relative to each other. A “shear acoustic mode” is anacoustic vibration mode in a medium that results in shear deformation ofthe medium. The shear deformations in the XBAR 400 are represented bythe curves 460, with the adjacent small arrows providing a schematicindication of the direction and magnitude of atomic motion. The degreeof atomic motion, as well as the thickness of the piezoelectric plate410, have been greatly exaggerated for ease of visualization. While theatomic motions are predominantly lateral (i.e. horizontal as shown inFIG. 4), the direction of acoustic energy flow of the excited primaryshear acoustic mode is substantially orthogonal to the surface of thepiezoelectric plate, as indicated by the arrow 465.

An acoustic resonator based on shear acoustic wave resonances canachieve better performance than current state-of-the artfilm-bulk-acoustic-resonators (FBAR) and solidly-mounted-resonatorbulk-acoustic-wave (SMR BAW) devices where the electric field is appliedin the thickness direction. In such devices, the acoustic mode iscompressive with atomic motions and the direction of acoustic energyflow in the thickness direction. In addition, the piezoelectric couplingfor shear wave XBAR resonances can be high (>20%) compared to otheracoustic resonators. High piezoelectric coupling enables the design andimplementation of microwave and millimeter-wave filters with appreciablebandwidth.

FIG. 5 is a schematic circuit diagram and layout for a high frequencyband-pass filter 500 using XBARs. The filter 500 has a conventionalladder filter architecture including three series resonators 510A, 510B,510C and two shunt resonators 520A, 520B. The three series resonators510A, 510B, and 510C are connected in series between a first port and asecond port (hence the term “series resonator”). In FIG. 5, the firstand second ports are labeled “In” and “Out”, respectively. However, thefilter 500 is bidirectional and either port may serve as the input oroutput of the filter. The two shunt resonators 520A, 520B are connectedfrom nodes between the series resonators to ground. A filter may containadditional reactive components, such as inductors, not shown in FIG. 5.All the shunt resonators and series resonators are XBARs. The inclusionof three series and two shunt resonators is exemplary. A filter may havemore or fewer than five total resonators, more or fewer than threeseries resonators, and more or fewer than two shunt resonators.Typically, all of the series resonators are connected in series betweenan input and an output of the filter. All of the shunt resonators aretypically connected between ground and the input, the output, or a nodebetween two series resonators.

In the exemplary filter 500, the three series resonators 510A, B, C andthe two shunt resonators 520A, B of the filter 500 are formed on asingle plate 530 of piezoelectric material bonded to a silicon substrate(not visible). Each resonator includes a respective IDT (not shown),with at least the fingers of the IDT disposed over a cavity in thesubstrate. In this and similar contexts, the term “respective” means“relating things each to each”, which is to say with a one-to-onecorrespondence. In FIG. 5, the cavities are illustrated schematically asthe dashed rectangles (such as the rectangle 535). In this example, eachIDT is disposed over a respective cavity. In other filters, the IDTs oftwo or more resonators may be disposed over a single cavity.

Each of the resonators 510A, 510B, 510C, 520A, 520B in the filter 500has a resonance where the admittance of the resonator is very high andan anti-resonance where the admittance of the resonator is very low. Theresonance and anti-resonance occur at a resonance frequency and ananti-resonance frequency, respectively, which may be the same ordifferent for the various resonators in the filter 500. Inover-simplified terms, each resonator can be considered a short-circuitat its resonance frequency and an open circuit at its anti-resonancefrequency. The input-output transfer function will be near zero at theresonance frequencies of the shunt resonators and at the anti-resonancefrequencies of the series resonators. In a typical filter, the resonancefrequencies of the shunt resonators are positioned below the lower edgeof the filter's passband and the anti-resonance frequencies of theseries resonators are position above the upper edge of the passband.

FIG. 6 is a schematic cross-sectional view through a shunt resonator anda series resonator of a filter 600 that uses a dielectric frequencysetting layer to separate the resonance frequencies of shunt and seriesresonators. A piezoelectric plate 610 is attached to a substrate 620.Portions of the piezoelectric plate 610 form diaphragms spanningcavities 640 in the substrate 620. Interleaved IDT fingers, such asfinger 630, are formed on the diaphragms. A first dielectric layer 650,having a thickness t1, is formed over the IDT of the shunt resonator.The first dielectric layer 650 is considered a “frequency settinglayer”, which is a layer of dielectric material applied to a firstsubset of the resonators in a filter to offset the resonance frequenciesof the first subset of resonators with respect to the resonancefrequencies of resonators that do not receive the dielectric frequencysetting layer. The dielectric frequency setting layer is commonly SiO₂but may be silicon nitride, aluminum oxide, or some other dielectricmaterial. The dielectric frequency setting layer may be a laminate orcomposite of two or more dielectric materials.

A second dielectric layer 655, having a thickness t2, may be depositedover both the shunt and series resonator. The second dielectric layer655 serves to seal and passivate the surface of the filter 600. Thesecond dielectric layer may be the same material as the first dielectriclayer or a different material. The second dielectric layer may be alaminate or composite of two or more different dielectric materials.Further, as will be described subsequently, the thickness of the seconddielectric layer may be locally adjusted to fine-tune the frequency ofthe filter 600A. Thus, the second dielectric layer can be referred to asthe “passivation and tuning layer”.

The resonance frequency of an XBAR is roughly proportional to theinverse of the total thickness of the diaphragm including thepiezoelectric plate 610 and the dielectric layers 650, 655. Thediaphragm of the shunt resonator is thicker than the diaphragm of theseries resonator by the thickness t1 of the dielectric frequency settinglayer 650. Thus, the shunt resonator will have a lower resonancefrequency than the series resonator. The difference in resonancefrequency between series and shunt resonators is determined by thethickness t1.

Lithium niobate and lithium tantalate are preferred piezoelectricmaterials for use in XBARs. The term “lithium niobate XBAR” means anXBAR with a lithium niobate diaphragm. Similarly, the term “lithiumtantalate XBAR” means an XBAR with a lithium tantalate diaphragm.Lithium niobate XBARs have very high electromechanical coupling, whichresults in a large difference between the resonance and anti-resonancefrequencies of a lithium niobate XBAR. Lithium niobate XBARs aresuitable for use in bandpass filters for ultra-wide communications bandssuch as band N77 (3400 to 4300 MHz) and band N79 (4400 to 5000 MHz).However, lithium niobate has a temperature coefficient of frequency(TCF) of about −80 to ˜100 ppm/C° (parts per million per degreeCelsius). The passband of filters using lithium niobate XBARs may shiftin frequency by more than 1% over the operating temperature range of acommunications device.

Lithium tantalate XBARs have lower electromechanical coupling thanlithium niobate XBARs, which results in a smaller difference between theresonance and anti-resonance frequencies of a lithium tantalate XBAR. Itis not practical to design filters for band N77 or band N79 using onlylithium tantalate XBARs. However, the TCF of lithium tantalate XBARs isroughly half of the TCF of lithium niobate XBARs. Typically, the TCF oflithium niobate is about 80 ppm per degree Celsius; the TCF of lithiumniobate is about 40 ppm per degree Celsius.

Using both lithium niobate XBARs and lithium tantalate XBARs in the samefilter provides for the best features of each technology. Such filterscan have the wide bandwidth enabled by the large electromechanicalcoupling of lithium niobate XBARs and the low TCF of lithium tantalateXBARs.

FIG. 7 is a schematic diagram of a bandpass filter 700 that takesadvantage of the features of both lithium niobate and lithium tantalateXBARs. The filter 700 includes five series resonators S1, S2, S3, S4,and S5 and four shunt resonators P1, P2, P3, and P4 connected in ladderfilter circuit. Shunt resonators P2 and P3 are lithium tantalate XBARs.The other resonators are lithium niobate XBARs. The rationale for thisdesign can be appreciated upon consideration of FIG. 8.

FIG. 8 is a graph of the input/output transfer function of the bandpassfilter 700 and the admittances of the four shunt resonators P1, P2, P3,P4 within the filter 700. The data in FIG. 7 and subsequent figures isbased on simulation of XBAR filters using a finite element method.Specifically, the bold curve 810 is the input/output transfer functionof the filter 700, which is read using the right-hand vertical scale.The solid curves 820, 825 are the admittances of resonators P1 and P4,which are lithium niobate XBARs. The dashed curves 830, 835 are theadmittances of resonators P2 and P3, which are lithium tantalate XBARs.Curves 820, 825, 830, 835 are read using the left-hand vertical scale.

As previously explained, the input-output transfer function of a filterwill be near zero at the resonance frequencies of the shunt resonatorsand at the anti-resonance frequencies of the series resonators. In otherwords, each shunt resonator creates a “transmission zero” at itsresonance frequency. Shunt resonators P2 and P3 (dashed curves 830, 835)create transmission zeros proximate the lower edge of the filterpassband. In particular, the resonator represented by the dashed curve830 (it does not matter whether this is P2 or P3) creates a transmissionzero immediately adjacent to the lower edge of the passband.“Immediately adjacent” means “adjacent with nothing else (i.e. no othertransmission zero) intervening”. This resonator defines, or sets, thelower edge of the passband. Typically, a single shunt resonator definesthe lower edge of a filter's passband and a single series resonatordefines the upper edge of the passband. In some cases, two or moreresonators may create transmission zeros at the same frequency andjointly define the upper or lower band edge.

For the filter 700, the TCF of the lower edge of the passband will beapproximately equal to the TCF of the resonator that defines the lowerband edge (i.e. the resonator that creates the transmission zeroimmediately adjacent to the band edge). Since resonators P2 and P3 arelithium tantalate XBARs, the TCF of the lower edge of the passband willbe about equal to the TCF of lithium tantalate. The benefit of using alithium tantalate XBAR to define the lower edge of the passband isillustrated in FIG. 9.

FIG. 9 is a graph 900 of the input/output transfer function of thefilter 700 of FIG. 7 at two widely separated temperatures. Specifically,the solid curve 910 is a plot of the input/output transfer function ofthe filter 700 at a high temperature. The dashed curve 920 is a plot ofthe input/output transfer function of the filter 700 at a lowtemperature. The difference between the high and low temperatures is 115degrees Celsius. The lower edge of the passband, which is defined bylithium tantalate XBARs, shifts by about 0.6% of the center frequencyover this temperature range. The upper edge of the pass band, which isdefined by lithium niobate XBARs, shifts by about 1.4% of the centerfrequency over the same temperature range.

FIG. 10 is a schematic diagram of another bandpass filter 1000 thattakes advantage of the features of both lithium niobate and lithiumtantalate XBARs. The filter 1000 includes five series resonators S1, S2,S3, S4, and S5 and four shunt resonators P1, P2, P3, and P4. Seriesresonators S2, S3, and S4 are lithium tantalate XBARs. the otherresonators are lithium niobate XBARs.

The input-output transfer function of the filter 1000 will be near zeroat the anti-resonance frequencies of the series resonators. In otherwords, each series resonator creates a “transmission zero” at itsanti-resonance frequency. At least one of the lithium tantalate seriesresonators S2, S3, S4 defines the upper edge of the passband of thefilter by creating at least one transmission zeros immediately adjacentthe upper edge of the filter passband. In some cases, two or more seriesresonators may create transmission zeros at the same frequency andjointly define the band edge. The TCF of the upper edge of the passbandwill be approximately equal to the TCF of the resonator that creates theimmediately adjacent transmission zero. Since the band edge is definedby one or more of the lithium tantalate XBAR series resonators S2, S3,S4, the TCF of the upper edge of the passband will be about equal to theTCF of lithium tantalate. The benefit of using a lithium tantalate XBARto define the upper edge of the passband is illustrated in FIG. 11.

FIG. 11 is a graph 1100 of the input/output transfer function of thefilter 1000 of FIG. 10 at two widely separated temperatures.Specifically, the solid curve 1110 is a plot of the input/outputtransfer function of the filter 1000 at a high temperature. The dashedcurve 1120 is a plot of the input/output transfer function of the filter1000 at a low temperature. The difference between the high and lowtemperatures is 128 degrees Celsius. The upper edge of the passband,which is defined by lithium tantalate XBARs, shifts by about 0.6% of thecenter frequency over this temperature range. The lower edge of the passband, which is defined by lithium niobate XBARs, shifts by about 1.2% ofthe center frequency over the same temperature range.

FIG. 12 is a schematic diagram of another bandpass filter 1200 thattakes advantage of the features of both lithium niobate and lithiumtantalate XBARs. The filter 1200 includes five series resonators S1, S2,S3, S4, and S5 and four shunt resonators P1, P2, P3, and P4. Shuntresonators P2 and P3 are lithium tantalate XBARs. Series resonators S2,S3, and S4 are also lithium tantalate XBARs. The other resonators S1,P1, P4, S5 are lithium niobate XBARs.

The lower band edge of the bandpass filter 1200 is defined by one ormore of the lithium tantalate XBAR shunt resonators P2, P3. Thus, thechange in the frequency of the lower band edge with temperature will besimilar to that of the filter 700 as shown in FIG. 9. The upper bandedge of the bandpass filter 1200 is defined by one or more of thelithium tantalate XBAR series resonators S2, S3, S4. Thus, the change inthe frequency of the upper band edge with temperature will be similar tothat of the filter 1000 as shown in FIG. 11.

FIG. 13A is an exemplary schematic plan view of a filter 1300, which hasthe same schematic diagram as the ladder filter 1200 of FIG. 12. Lithiumtantalate XBARs P2, P3, S2, S3, and S4 are fabricated on a first chip1310. Lithium niobate XBARs P1, P4, S1, and S5 are fabricated on asecond chip 1320. Within each chip 1310, 1320, the XBARs areinterconnected by conductors, such as conductor 1330, formed on therespective chip. The chips 1310, 1320 are electrically connected to eachother and to a system external to the filter by means of pads, such aspad 1340. Each pad may, for example, be, or interface with, a solder orgold bump to connect with a circuit card (not shown).

Electrical connections 1350 between the lithium tantalate XBARs on thefirst chip 1310 and the lithium niobate XBARs on the second chip 1320are shown as bold lines. The connections 1350 are made, for example, byconductors on a circuit card to which the first and second chips aremounted. In this context, the term “circuit card” means an essentiallyplanar structure containing conductors to connect the first and secondchips to each other and to a system external to the band-pass filter1300. The circuit card may be, for example, a single-layer ormulti-layer printed wiring board, a low temperature co-fired ceramic(LTCC) card, or some other type of circuit card. Traces on the circuitcard can have very low resistance such that losses in the traces arenegligible. The inductance of the electrical connections 1350 betweenthe series and shunt resonators can be compensated in the design of theacoustic wave resonators. In some cases, the inductance of theelectrical connections 1350 can be exploited to improve the performanceof the filter, for example by lowering the resonance frequency of one ormore shunt resonators to increase the filter bandwidth.

FIG. 13B is a schematic cross-sectional view of the bandpass filter1300. The split ladder filter 1300 includes a first chip 1310 and asecond chip 1320 attached to, and interconnected by, a circuit card1360. In this example, the first and second chips 1310, 1320 are“flip-chip” mounted to the circuit card 1360. Electrical connectionsbetween the first and second chips, 1310, 1320 and the circuit card 1360are made by solder or gold bumps, such as bump 1370. Electricalconnections between the first chip 1310 and the second chip 1320 aremade by conductors on or within the circuit card 1360. The first andsecond chips 1310, 1320 may be mounted on and/or connected to thecircuit card 1360 in some other manner.

The filters 700, 1000, and 1200 are exemplary. A bandpass filter mayhave more or fewer than nine resonators, more or fewer than four shuntresonators, and more or fewer than five series resonators. A filterusing both lithium tantalate XBARs and lithium niobate XBARs will haveat least one lithium tantalate XBAR, which may be shunt resonator or aseries resonator, and at least one lithium niobate XBAR, which may beshunt resonator or a series resonator. One or both of the upper bandedge and the lower band edge may be defined by lithium tantalate XBARs.

Description of Methods

FIG. 14 is a simplified flow chart showing a process 1400 for making anXBAR or a filter incorporating XBARs. The process 1400 starts at 1405with a substrate and a plate of piezoelectric material and ends at 1495with a completed XBAR or filter. The flow chart of FIG. 14 includes onlymajor process steps. Various conventional process steps (e.g. surfacepreparation, cleaning, inspection, baking, annealing, monitoring,testing, etc.) may be performed before, between, after, and during thesteps shown in FIG. 14.

The flow chart of FIG. 14 captures three variations of the process 1400for making an XBAR which differ in when and how cavities are formed inthe substrate. The cavities may be formed at steps 1410A, 1410B, or1410C. Only one of these steps is performed in each of the threevariations of the process 1400.

The piezoelectric plate may be lithium niobate or lithium tantalate. Thepiezoelectric plate may be Z-cut, rotated Z-cut, or rotated YX-cut, orsome other cut. The substrate may preferably be silicon. The substratemay be some other material that allows formation of deep cavities byetching or other processing.

In one variation of the process 1400, one or more cavities are formed inthe substrate at 1410A, before the piezoelectric plate is bonded to thesubstrate at 1420. A separate cavity may be formed for each resonator ina filter device. The one or more cavities may be formed usingconventional photolithographic and etching techniques. Typically, thecavities formed at 1410A will not penetrate through the substrate, andthe resulting resonator devices will have a cross-section as shown inFIG. 3.

At 1420, the piezoelectric plate is bonded to the substrate. Thepiezoelectric plate and the substrate may be bonded by a wafer bondingprocess. Typically, the mating surfaces of the substrate and thepiezoelectric plate are highly polished. One or more layers ofintermediate materials, such as an oxide or metal, may be formed ordeposited on the mating surface of one or both of the piezoelectricplate and the substrate. One or both mating surfaces may be activatedusing, for example, a plasma process. The mating surfaces may then bepressed together with considerable force to establish molecular bondsbetween the piezoelectric plate and the substrate or intermediatematerial layers.

A conductor pattern, including IDTs of each XBAR, is formed at 1430 bydepositing and patterning one or more conductor layer on the front sideof the piezoelectric plate. The conductor layer may be, for example,aluminum, an aluminum alloy, copper, a copper alloy, or some otherconductive metal. Optionally, one or more layers of other materials maybe disposed below (i.e. between the conductor layer and thepiezoelectric plate) and/or on top of the conductor layer. For example,a thin film of titanium, chrome, or other metal may be used to improvethe adhesion between the conductor layer and the piezoelectric plate. Aconduction enhancement layer of gold, aluminum, copper or other higherconductivity metal may be formed over portions of the conductor pattern(for example the IDT bus bars and interconnections between the IDTs).

The conductor pattern may be formed at 1430 by depositing the conductorlayer and, optionally, one or more other metal layers in sequence overthe surface of the piezoelectric plate. The excess metal may then beremoved by etching through patterned photoresist. The conductor layercan be etched, for example, by plasma etching, reactive ion etching, wetchemical etching, and other etching techniques.

Alternatively, the conductor pattern may be formed at 1430 using alift-off process. Photoresist may be deposited over the piezoelectricplate. and patterned to define the conductor pattern. The conductorlayer and, optionally, one or more other layers may be deposited insequence over the surface of the piezoelectric plate. The photoresistmay then be removed, which removes the excess material, leaving theconductor pattern.

At 1440, a front-side dielectric layer or frequency setting dielectriclayer may be formed by depositing one or more layers of dielectricmaterial on the front side of the piezoelectric plate. The one or moredielectric layers may be deposited using a conventional depositiontechnique such as sputtering, evaporation, or chemical vapor deposition.The one or more dielectric layers may be deposited over the entiresurface of the piezoelectric plate, including on top of the conductorpattern. Alternatively, one or more lithography processes (usingphotomasks) may be used to limit the deposition of the dielectric layersto selected areas of the piezoelectric plate, such as only between theinterleaved fingers of the IDTs. Masks may also be used to allowdeposition of different thicknesses of dielectric materials on differentportions of the piezoelectric plate.

In a second variation of the process 1400, one or more cavities areformed in the back side of the substrate at 1410B. A separate cavity maybe formed for each resonator in a filter device. The one or morecavities may be formed using an anisotropic or orientation-dependent dryor wet etch to open holes through the back-side of the substrate to thepiezoelectric plate. In this case, the resulting resonator devices willhave a cross-section as shown in FIG. 1.

In a third variation of the process 1400, one or more cavities in theform of recesses in the substrate may be formed at 1410C by etching thesubstrate using an etchant introduced through openings in thepiezoelectric plate. A separate cavity may be formed for each resonatorin a filter device. The one or more cavities formed at 1410C will notpenetrate through the substrate, and the resulting resonator deviceswill have a cross-section as shown in FIG. 3.

In all variations of the process 1400, the filter device is completed at1460. Actions that may occur at 1460 include depositing anencapsulation/passivation layer such as SiO₂ or Si₃O₄ over all or aportion of the device; forming bonding pads or solder bumps or othermeans for making connection between the device and external circuitry;excising individual devices from a wafer containing multiple devices;other packaging steps; and testing. Another action that may occur at1460 is to tune the resonant frequencies of the resonators within thedevice by adding or removing metal or dielectric material from the frontside of the device. After the filter device is completed, the processends at 1495.

FIG. 15 is a flow chart of a method 1500 for fabricating a filter usinglithium niobate and lithium tantalate XBARs, which may be the filters700, 1000, or 1200. The method 1500 starts at 1510 and concludes at 1590with a completed filter device.

At 1520, a first chip containing one or more XBAR resonators withlithium tantalate diaphragms is fabricated. The first chip may befabricated using the process 1400 with a lithium tantalate piezoelectricplate. The first chip contains a subset of the resonators of the filterdevice, which may include series resonators and/or shunt resonators. Thefirst chip may be a portion of a first large multi-chip wafer such thatmultiple copies of the first chip are produced during each repetition ofthe step 1520. In this case, individual chips may be excised from thewafer and tested as part of the action at 1520.

At 1530, a second chip containing one or more XBAR resonators withlithium niobate diaphragms is fabricated. The second chip may befabricated using the process 1400 using a lithium niobate piezoelectricplate. The second chip contains a subset of the resonators of the filterdevice, which may include series resonators and/or shunt resonators. Thesecond chip may be a portion of a second large multi-chip wafer suchthat multiple copies of the second chip are produced during eachrepetition of the step 1530. In this case, individual chips may beexcised from the wafer and tested as part of the action at 1530.

At 1540, a circuit card is fabricated. The circuit card may be, forexample, a printed wiring board or an LTCC card or some other form ofcircuit card. The circuit card may include one or more conductors formaking at least one electrical connection between a resonator on thefirst chip and a resonator on the second chip. The circuit may be aportion of large substrate such that multiple copies of the circuit cardare produced during each repetition of the step 1540. In this case,individual circuit cards may be excised from the substrate and tested aspart of the action at 1540. Alternatively, individual circuit cards maybe excised from the substrate after chips have been attached to thecircuit cards at 1550, or after the devices are packaged at 1560.

At 1550, individual first and second chips are assembled to a circuitcard (which may or may not be a portion of a larger substrate) usingknown processes. For example, the first and second chips may be“flip-chip” mounted to the circuit card using solder or gold bumps orballs to make electrical, mechanical, and thermal connections betweenthe chips and the circuit card. The first and second chips may beassembled to the circuit card in some other manner.

The filter device is completed at 1560. Completing the filter device at1560 includes packaging and testing. Completing the filter device at1560 may include excising individual circuit card/chip assemblies from alarger substrate before or after packaging.

CLOSING COMMENTS

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. A filter having a passband between a lower band edgeand an upper band edge, comprising: a plurality of transversely-excitedfilm bulk acoustic resonators (XBARs) connected in a ladder filtercircuit, wherein the plurality of XBARs includes at least one lithiumtantalate (LT) XBAR and at least one lithium niobate (LN) XBAR, thelower band edge is defined by one or more LT XBARs from the plurality ofXBARs, and the upper band edge is defined by one or more LT XBARs fromthe plurality of XBARs.
 2. The filter of claim 1, wherein the one ormore LT XBARS are shunt resonators in the ladder filter circuit.
 3. Thefilter of claim 1, wherein the one or more LT XBARS are seriesresonators in the ladder filter circuit.
 4. The filter of claim 1,wherein the lower band edge is defined by a first LT XBAR from theplurality of XBARs and the upper band edge is defined by a second LTXBAR from the plurality of XBARs.
 5. The filter of claim 4, wherein thefirst LT XBAR is a shunt resonator in the ladder filter circuit and thesecond LT XBAR is a series resonator in the ladder filter circuit.
 6. Afilter having a passband between a lower band edge and an upper bandedge, comprising: a first chip comprising one or more lithium tantalate(LT) transversely-excited film bulk acoustic resonators (XBARs); and asecond chip comprising one or more lithium niobate (LN) XBARs, whereinthe one or more LT XBARs and the one or more LN XBARs are connected toform a ladder filter circuit.
 7. The filter of claim 6, wherein thefirst chip comprises: a first substrate having a surface; an LT plate, aback surface of the LT plate attached to the surface of the firstsubstrate, portions of the LT plate forming one or more LT diaphragmsspanning cavities in the first substrate; and a first conductor patternformed on a front surface of the LT plate, the first conductor patternincluding a respective interdigital transducer (IDT) of each of the oneor more LT XBARs, interleaved fingers of each IDT disposed on arespective one of the one or more LT diaphragms.
 8. The filter of claim6, wherein the second chip comprises: a second substrate having asurface; an LN plate, a back surface of the LN plate attached to thesurface of the second substrate, portions of the LN plate forming one ormore LN diaphragms spanning cavities in the second substrate; and asecond conductor pattern formed on a front surface of the LN plate, thesecond conductor pattern including a respective interdigital transducer(IDT) of each of the one or more LN XBARs, interleaved fingers of eachIDT disposed on a respective one of the one of more LN diaphragms. 9.The filter of claim 6, wherein the lower band edge is defined by a firstLT XBAR of the one or more LT XBARs.
 10. The filter of claim 9, whereinthe first LT XBAR is a shunt resonator in the ladder filter circuit. 11.The filter of claim 6, wherein the lower band edge is defined by a firstLT XBAR of the one or more LT XBARs
 12. The filter of claim 11, whereinfirst LT XBAR is a series resonator in the ladder filter circuit. 13.The filter of claim 6, wherein the one or more LT XBARs is at least twoLT XBARs, the lower band edge is defined by a first LT XBAR of the atleast two LT XBARs, and the upper band edge is defined by a second LTXBAR of the at least two LT XBARs.
 14. The filter of claim 13, whereinthe first LT XBAR is a shunt resonator in the ladder filter circuit andthe second LT XBAR is a series resonator in the ladder filter circuit.15. The filter of claim 6, the connection comprising: at least oneconductor for connecting an LT XBAR of the first chip to an LN XBAR ofthe second chip.
 16. A method for fabricating a filter, comprising:fabricating a first chip containing one or more lithium tantalate (LT)transversely-excited film bulk acoustic resonators (XBARs); fabricatinga second chip containing one or more lithium niobate (LN) XBARs; andconnecting the first chip to the second chip, wherein connectingcomprises at least one conductor for connecting an LT XBAR of the firstchip to an LN XBAR of the second chip.
 17. The method of claim 16,wherein the first chip comprises: a first substrate having a surface; anLT plate, a back surface of the LT plate attached to the surface of thefirst substrate, portions of the LT plate forming one or more LTdiaphragms spanning cavities in the first substrate; and a firstconductor pattern formed on a front surface of the LT plate, the firstconductor pattern including a respective interdigital transducer (IDT)of each of the one or more LT XBARs, interleaved fingers of each IDTdisposed on a respective one of the one or more LT diaphragms.
 18. Themethod of claim 16, wherein the second chip comprises: a secondsubstrate having a surface; an LN plate, a back surface of the LN plateattached to the surface of the second substrate, portions of the LNplate forming one or more LN diaphragms spanning cavities in the secondsubstrate; and a second conductor pattern formed on a front surface ofthe LN plate, the second conductor pattern including a respectiveinterdigital transducer (IDT) of each of the one or more LN XBARs,interleaved fingers of each IDT disposed on a respective one of the oneof more LN diaphragms.
 19. The method of claim 16, wherein the lowerband edge is defined by a first LT XBAR of the one or more LT XBARs. 20.The method of claim 19, wherein the first LT XBAR is a shunt resonatorin the ladder filter circuit.
 21. The method of claim 16, wherein thelower band edge is defined by a first LT XBAR of the one or more LTXBARs
 22. The method of claim 21, wherein first LT XBAR is a seriesresonator in the ladder filter circuit.
 23. The method of claim 16,wherein the one or more LT XBARs is at least two LT XBARs, the lowerband edge is defined by a first LT XBAR of the at least two LT XBARs,and the upper band edge is defined by a second LT XBAR of the at leasttwo LT XBARs.
 24. The method of claim 23, wherein the first LT XBAR is ashunt resonator in the ladder filter circuit and the second LT XBAR is aseries resonator in the ladder filter circuit.
 25. The method of claim16, the connection comprising: at least one conductor for connecting anLT XBAR of the first chip to an LN XBAR of the second chip.